THE VARIOUS FLIGHT MODES

It appears that there will be three radically different flight modes in which a Steam Balloon may be operated:

(1) The naked dribble mode;

(2) The jacketed dribble mode;

(3) The jacketed reboiling mode;

(There is no practical naked reboiling mode)

(And a steam airship, which is far in the future, would only be able to operate in the jacketed reboiling mode)

Each of these modes will have its advantages and drawbacks. Before gaining flight experience, it is difficult to predict or to quantify the exact pros and cons of each mode. The following tentative analysis, which attempts to err (if at all) on the pessimistic side, is based upon the numerical values determined from the experiments we have performed so far, and upon the facts of basic physics.

DEFINITION OF TERMS

By "naked" is meant that no insulation jacket is carried upon the balloon envelope, which accordingly is exposed directly to the atmosphere, and thus condenses steam comparatively rapidly;

By "dribble" is meant that the condensed water trickling down the envelope is immediately discharged into the atmosphere and released - thus in such a mode no burner or boiler is carried during flight; and

By "reboiling " is meant that the condensed water is not discharged, but is retained and boiled back into steam by a flight burner/boiler carried in the basket and controlled by the pilot.

These flight modes will be discussed in terms of various exemplary balloon envelopes. It should be understood that in all cases the initial filling of the balloon on the ground will be done from a large capacity ground-based boiler, presumably a mobile one for field deployment.

ENVELOPE (A)
SMALL SOLO BALLOON

First we consider a small envelope of 600 m3 volume and 400 m2 area, weighing 40 kg including load tapes, vent, etc. The gross lift (at 6.26 N/m2) will be 380 kg and the parasitic trickling-down water (at 75 gm/m2) can reliably be estimated as 30 kg. This balloon is similar to the one we have already constructed, described here. We allow 30 kg for a seat, arrangements for holding and dropping water ballast, and supporting lines, and 80 kg for the solo pilot.

The lift net of the above elements therefore will be:

380 kg...........gross
-40 kg...........envelope
-30 kg...........parasitic water
-30 kg...........seat, lines, etc.
-80 kg...........pilot
-------------------------
200 kg...........remaining net lift
-------------------------

INSULATION JACKET (A)

Moreover, to consider the insulation jacket (for those modes in which one is fitted), it appears from our experiments that an insulating jacket 3 cm thick made of 200 gm/m2 Primaloft PL1, weighing about 300 gm/m3 with batting etc., which seems like a good compromise between weight and bulk versus efficiency, will bring the condensation of water down from the naked value of 1.4 kg/m2.hr to 200 gm/m2.hr and perhaps rather less. For the envelope (A) above, such a jacket will weigh 120 kg and will bulk 12 m3.

PERFORMANCE ANALYSIS

(A1) SMALL SOLO BALLOON
NAKED DRIBBLE MODE

On the launch site, this balloon is connected (naked, without any insulation jacket) to the high-capacity ground boiler and is filled with steam. When full, the net buoyancy will be 200 kg as detailed above. 200 kg of ballast are taken on board to establish substantially neutral buoyancy, the craft is released, and the flight commences. As water condenses from the steam lift gas and trickles down to the base of the envelope, it drains off and is immediately discharged.

Here is a rather crude figure showing the principle of this naked dribble mode:

For this first envelope, the rate of steam condensation is about 560 kilos per hour - nearly 10 kg/minute, and the condensation of one kilo of steam into water which is discharged entails the loss of almost exactly one kilo of lift (see the basic physics page), Therefore about 10 kg/minute of ballast must also be released in order to maintain neutral buoyancy, so that the maximum flight duration comes out at about 20 minutes. (Actually rather longer because the envelope effective area, and the resultant condensation rate, will diminish as the steam volume becomes less.)

Although time-limited, such a flight will have a peculiar charm. The rate of release of ballast to maintain neutral flight is very great by gas ballooning standards - nearly 10 kg/min - no wonder this is termed the "dribble mode" of flight! (better than the "pissing mode") Some form of automatic ballast release may be desirable, with the pilot making fine adjustments for height control. So much ballast is carried that the capability for vertical control may be the greatest of any flight means in history! Since there is no flight burner/boiler, silence will be absolute. The on-board flight equipment is also of ultimate simplicity - nothing but ballast.

This naked dribble flight mode has the advantage that there is no problem of envelope bulk. Upon landing it is only necessary to drain off the surplus water lingering inside the envelope - perhaps 50 kg or so - which can easily be done by turning it inside-out. Then the balloon can be packed away tight and small, like any hot-air balloon.

However, the ground boiler for initially filling the envelope with steam before takeoff needs to be fairly powerful, due to the high rate of condensation of a naked envelope. For example, a ground boiler which produced 500 kg/hour of steam could never inflate even this small balloon, because it could never catch up with the condensation. A one-ton per hour boiler (burning perhaps 100 kg/hour of fuel) could get the job done in an hour; this is probably the minimum practical ground boiler size. The technology employed for the ground boiler can be completely conventional, since weight and efficiency are only minor issues.

FLYING THIS SMALL BALLOON WITHOUT REGISTRATION

This small balloon can actually be flown in the USA in the naked dribbling mode (only) under the FAR-103 rule, i.e. with no registration, no pilot licence, and no paperwork of any sort, because its ground weight excluding ballast will not exceed 70 kg. The matter is quite clear from this quotation, although the rule was not written with this type of flight in mind. Here is the official FAR-103 preamble and here is the text; commentaries bringing out what is possible may be seen here and here.

(A2) SMALL SOLO BALLOON
JACKETED DRIBBLE MODE

Suppose now that the above envelope (A) is fitted with the insulating jacket described above, which weighs 120 kg, so that now only 80 kg of reserve buoyancy is available at takeoff. As before, variation of the rate of ballast discharge provides vertical control. The condensation rate is now only 80 kilos of steam per hour, so that the maximum flight duration is now about one hour or a bit more; as we have discovered, the efficiency of the insulating jacket increases substantially as the underlying envelope becomes slacker.

Here is an illustration explaining this jacketed dribble mode:

This jacketed dribble flight mode is subject to the disadvantage of craft bulk. The volume of the insulation jacket is 12 m3, so that the problem of packing it away after landing is substantial, even though it is not very heavy. Obviously a purpose-built trailer is required. However, there is the interesting possibility of vacuum-packing the jacket. The currently favorite jacket material, Primaloft PL1, is specifically designed to be squeezed down very small - like a natural down sleeping bag - and to spring back when released. And a suitable vacuum bag is readily to hand - the envelope itself. According to this concept, after landing, one would turn the complete envelope with jacket on it inside out, so that the jacket was now on the inside and the envelope on the outside, and would then connect a small air pump to suck out the air from the envelope. Primaloft is specified to crush down 95% under 30 kpascals of vacuum, so in theory the volume would become quite manageable. Without actually trying it, it is hard to say how this procedure would work out in practice.... usually the case with novel LTA ground handling arrangements...

A somewhat smaller ground boiler would be sufficient for initially filling this insulated envelope before takeoff. A 600 kg per hour boiler (burning 60 kg/hour of fuel) would take about an hour.

(A3) SMALL SOLO BALLOON
JACKETED REBOILING MODE

The dribble mode of flight - with or without an insulating jacket - has the advantage of simplicity, and it is probable that our first Steam Balloon flights will be dribble ones. However, with an insulation jacket this small balloon (A) has 80 kg of extra buoyancy available at takeoff. Rather than, as in the dribble modes, discharging the water condensed from the steam lift gas and using the initial buoyancy for carrying ballast, there is the possibility of carrying a flight boiler and fuel for reboiling the condensed water back into steam: this is the reboiling mode of flight. Vertical control is performed by varying the rate of boiler operation, so that the amount of reserve water is either increased or reduced.

Here is an illustration explaining this jacketed reboiling mode:

For steady-state flight with this envelope (A) and its insulating jacket, the flight boiler will have to reboil 80 kg/hour of condensed water back into steam, and this will require about 8 kg/hour of fuel. The question is: how light can the boiler and burner be built?

There is a discussion of lightweight boilers in my Friedrichshafen paper which is still generally valid. A summary is here. These are water-tube boilers which are not particularly easy to make. The art of lightweight boiler and burner making was brought to its highest level during the development of the steam car. As yet the massed brains of the steam car enthusiasts' world have not been brought to bear on the question of a flight boiler for the Steam Balloon; but when the time comes they will undoubtedly prove very fertile...

However, to be conservative, here we will assume the use of a fire-tube boiler of a conventional type, but built as light as possible. In this case, it seems that a reasonable guess at a burner/boiler all-up weight, for the above performance, might be about 40 kg. That leaves about 40 kg available for fuel, with this envelope, which would be enough for four or five hours flight. This is a superior performance as compared with a conventional hot-air balloon.

Strangely enough, rather than burning LPG, using solid fuel such as anthracite might be more effective. The furnace arrangements certainly would be simpler and the total installation would be lighter and safer. Emergency LPG injection might be provided for enabling abrupt upward acceleration.

ENVELOPE (B)
MEDIUM SOLO BALLOON

Next, let us consider a somewhat larger envelope (B), about 20% larger than envelope (A) in linear dimensions, and thus about 44% greater in area and 73% greater in volume. Thus the volume is 1037 m3 and the area is 580 m2, with the weight being say 60 kg. The gross lift will be 663 kg and the parasitic trickling-down water will be 43 kg. Again we allow 30 kg for the basket etc. and 80 kg for the solo pilot.

The lift net of the above elements therefore will be:

663 kg...........gross
-60 kg...........envelope
-43 kg...........parasitic water
-30 kg...........basket etc.
-80 kg...........pilot
-------------------------
450 kg...........remaining net lift
-------------------------

This is a bit more like it!

INSULATION JACKET (B}

By proportion, for this envelope (B), the insulation jacket will weigh (roughly) 180 kg and will bulk 18 m3.

PERFORMANCE ANALYSIS

(B1) MEDIUM SOLO BALLOON
NAKED DRIBBLE MODE

Upon initial filling, the net buoyancy will be 450 kg as detailed above, and so 450 kg of ballast will be taken on board. With this envelope (B), the rate of steam condensation will be about 800 kilos per hour - about 13.5 kg/minute. Therefore (following a calculation like the one for envelope A) the maximum flight duration will be a little over 30 minutes. This is not a great advance on envelope (A); the square-cube law doesn't seem to be working for us yet. The truth is that naked dribbling flight is not much use for anything other than short-duration joyrides..... the small envelope (A) is enough...

Again, there will be no problem of envelope bulk. A very powerful ground boiler will be needed for the initial fill, to get ahead of the condensation. A two-ton per hour boiler (burning 200 kg/hour of fuel) could comfortably do it in an hour.

(B2) MEDIUM SOLO BALLOON
JACKETED DRIBBLE MODE

Suppose that the above envelope (B) is fitted with its insulating jacket which weighs 180 kg, so that now 270 kg of reserve buoyancy is available at takeoff. The condensation rate is now 115 kilos of steam per hour, so that the maximum jacketed dribbling flight duration is now about two and a half hours. This is a pretty good result for such a low technology!

The bulk of the insulation jacket is now 18 m3. A one-ton per hour ground boiler (burning 100 kg/hour of fuel) would suffice, and would take about an hour for the initial fill.

(B3) MEDIUM SOLO BALLOON
JACKETED REBOILING MODE

With its insulation jacket this medium sized balloon (B) has 270 kg of extra buoyancy available at takeoff. If this is used for a flight boiler and fuel, rather than for ballast for dribbling flight, the question is: what performance can we expect?

For steady-state flight the flight boiler will have to reboil 115 kg/hour of condensed water back into steam, and this will require about 12 kg/hour of fuel. Going by scaling up the (hypothetical) numbers for envelope (A), the flight burner/boiler may weigh about 60 kg, which leaves 210 kg for fuel. That's enough for about 18 hours of flight! We see that the square-cube law is working hard for us. This performance is way beyond the hot-air balloon domain, and much more like a gas balloon - with hot air balloon controllability and at hot air balloon prices....

ENVELOPE (C)
LARGE MULTI-PERSON BALLOON

Next, let us consider a really large envelope (C), about 2X larger than envelope (A) in linear dimensions, and thus about 4X greater in area and 8X greater in volume. This'll get the square-cube law going all right! Thus the volume is 4800 m3 and the area is 1600 m2, with the weight being 160 kg. The gross lift will be 3040 kg and the parasitic trickling-down water will be 120 kg. Now we will allow 100 kg for the basket etc. and 250 kg for the pilot and three passengers.

The lift net of the above elements therefore will be:

3040 kg...........gross
-160 kg...........envelope
-120 kg...........parasitic water
-100 kg...........basket etc.
-250 kg...........pilot
-------------------------
2410 kg...........remaining net lift
-------------------------

This is a real balloon!

INSULATION JACKET (C}

By proportion, for this envelope (C), the insulation jacket will weigh (roughly) 480 kg and will bulk 48 m3.

PERFORMANCE ANALYSIS

(C1) LARGE MULTI-PERSON BALLOON
NAKED DRIBBLE MODE

Upon initial filling, the net buoyancy will be 2410 kg as detailed above, and so 2410 kg of ballast will be taken on board. With this envelope (C), the rate of steam condensation will be about 2240 kilos per hour - about 37 kg/minute. Therefore (following a calculation like the one for envelope A) the maximum flight duration will be a little over an hour. Even with this huge balloon, the advance on envelope (A) is not dazzling. This reinforces the conclusion that the potential of the naked dribbling flight mode is severely limited...

Again, there will be no problem of envelope bulk. An extremely powerful ground boiler will be needed for the initial fill - at least four tons per hour.

(C2) LARGE MULTI-PERSON BALLOON
JACKETED DRIBBLE MODE

Suppose now that the above envelope (C) is fitted with its insulating jacket which weighs 480 kg, so that 1930 kg of reserve buoyancy is available at takeoff. The condensation rate is now 320 kilos of steam per hour, so that the maximum jacketed dribbling flight duration is about six hours. This is actually an acceptable flight time for a full-day flight....

The bulk of the insulation jacket is now 48 m3. This will be a real animal to deal with on the ground. Vacuum compression will be almost essential.

(C3) LARGE MULTI-PERSON BALLOON
JACKETED REBOILING MODE

With its insulation jacket this large-sized balloon (C) has 1930 kg of extra buoyancy available at takeoff. If this is used for a flight boiler and fuel, the flight boiler will have to reboil 320 kg/hour of condensed water back into steam, and this will require about 35 kg/hour of fuel. Again going by scaling up the numbers for envelope (A), the flight burner/boiler may weigh about 160 kg, which leaves a whopping 1770 kg for fuel. That in theory is enough for 55 hours of flight, which implies that the simplistic flight plan "fly as long as you are able" is too extravagant to be retained. Anyway, we can see that the Steam Balloon technology has great promise in the larger sizes....



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